Pickup tube with dark current supply source



May 11, 1965 A. s. JENSEN ETAI.

PICKUP TUBE WITH DARK CURRENT SUPPLY SOURCE Filed May 3l, 1962 m .mi

m nml INVENTORS Arthur S. Jensen 8 Melvin P. Siedbond BY f i ELLd/AT: TRNQEYn/ United States Patent O Frice 3,133,400 PlClUl TUBE WITH DARK CURRENT SUPPLY SOURCE Arthur S. .leasen and Melvin P. Siedband, Baltimore, Md.,

assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed May 31, 1962, Ser. No. 198,984 o Claims. (Cl. 315-411) The present invention relates to pickup tubes, and, more particularly, to pickup tubes which utilize a return beam as a means for reading information deposited onto a retina or target structure.

Many pickup tubes and similar devices are based upon the principle that, when radiation impinges upon a target formed from a material known as a photoconductor, the conductivity thereof changes. When a voltage is impressed across this target material, the changes in conductivity produce changes in the amount of current which is permitted to lloW. In this Way, the iluctuations in the current correspond to the intensity of the impinging radiation. Some of these materials, particularly those which are responsive to radiations in a Wide frequency spectrum, for example, oxides of manganese, are inherently of low resistance. That is, in the absence of a radiation image upon the target, the resistance is relatively low but decreases further when irradiated. Thus, the application of a voltage to the non-irradiated target causes the flow of a so-called dark current Which may be of a relatively high amplitude.

When the target is irradiated by an image directed thereon, incremental areas of the target display a decrease in resistance in accordance with the intensity of the impingent radiation. This decrease in resistance results in an increase in current in these incremental areas to thus form a charge pattern on a surface thereof corresponding to the input radiation image. The surface is then scanned with an electron reading beam to read out the information which is stored on the surface in the form of this charge pattern. The surfaces of the layer of target material may be considered as plates of a capacitor, which, when charged, discharge through the material. If the resistance is low, the time constant of discharge may be so short that it does not store enough radiation information before it is read out by the reading beam. In ordinary vidicon types of camera tubes, this reading beam also recharges the target surface so that it may again accumulate and store new radiation information. Furthermore, because of the relatively large amplitude of the dark current, any fluctuations in the dark current may tend to mask the signal current and hence produce an undesirably low signal-to-noise ratio and consequently an undesirably low signal detectivity.

It is, therefore, an object of this invention to provide an improved pickup device.

A further object is to provide a pickup tube having an improved signal-to-noise ratio.

A further object is to provide a pickup tube which can use sensitive target materials of relatively low resistivity.

Another object is to provide an improved pickup tube which selectively collects electrons emanating from incremental areas of the target.

A still further object is to provide a pickup tube of improved detectivity.

Stated briefly, the present invention provides an improved pickup tube through the use of means for selectively collecting only those electrons, either primary elec- ,trons .'hich are returned or secondarily emitted electrons,

which emanate from the Vimmediate area of the target which is being scanned and interrogated by the tube reading beam at any particular instant. The preferred em- BSBA@ Patented May l1, 1965 bodiment of the device also includes a :second electron beam source which supplies the bulk of the dark current required by the target or retina.

ln actuality the electron beams do not necessarily supply the dark current or signal current but supply electrons to counteract the charge effected by these currents. However, for the sake of simplicity in this specification, the electron beams will be spoken of as supplying these currents.

Further objects and advantages of the invention Will become apparent as the following description proceeds and features of novelty which characterize the invention Will be pointed out in particularity in the claims annexed to and forming a part of this specification.

For a better understanding of the invention, reference may be had to the accompanying drawings, in which:

FlG. 1 is a diagrammatic illustration of the relationship existing between the various currents of a return beam pickup tube;

FIG. 2 is an elevational View in section of a pickup tube embodying the present invention; and

EYES. 3 and 4 are graphic illustrations showing typical secondary emission curves for various categories of material used as a target or retina.

The relationship between the various currents in a pickup tube may best be understood with respect to FIG. l. ln FIG. l, there is shown a target 33 whose resistivity or conductivity varies in accordance to the intensity of a radiation image impingent thereon. A beam of electrons ib is directed onto one surface of the target 33 from a suitable source such as an electron gun. In the absence of a radiation image on the target 33, the dark current id is supplied by the electron beam ib and the remainder of the beam is returned to a suitable output circuit. ln FlG. l, this return current or return beam is designated r. When a radiation image is impingent upon the target 33, the target resistance is reduced and the beam current ib supplies a signal current is which is subtracted from the return current ir of the non-irradiated target. The signal current is, which varies in accordance with the intensity of the radiation image upon the target 33, is ideally the only variable Within the tube and it is this variable effecting the intensity of the return beam fr which produces an intelligible reproducible signal in the output circuit.

In the usual pickup tube, the dark current id is large in comparison to the signal current is and as such constitutes a large portion of the total beam current it, Thus, small uctua-tions in d and/or electrons secondarily emitted from the target 33 and returning to the output circuit, may result in signals which are easily misinterpreted as signal current.

lit is known, in the prior ant, that a second electron beam source producing either a ood beam o-r a second scanning beam may be utilized to supply the necessary dark current. While this method may suitably be utilized to supply the dark current, the uncontrolled use of such a second beam source may produce disastrous results under improper conditions. If the dark current supplying beam is not impinigent upon the target at th-e proper energy level, the result may be that this beam will destroy the charge image thereon. It is also possible that this dark current electron beam will produce secondarily emitted electrons from the target structure -to such an extent that the noise iluctuations of the secondarily emitted electrons striking the output circuit will destroy or at least degrade the quality of the output image.

With reference now to FIG. 2, there is shown an image tube embodying the present invention in its preferred form. The device includes an .evacuated envelope 1t) which comprises an enlarged tubular portion 11 and a smaller tubular portion 12. The enlarged por-tion 11 may be closed ofi by means of a face plate 13 which is of a suitably wide band transmitting material such as sapphire, calcium iluoride, or barium iluoride, and which may be integrally formed with the envelope 1Q. The portion 12 is sealed oit and lmay be provided with a cap member 14, which is of a suitable material such as plastic, in a manner well known in the art.

A suitable electron beam producing source 15 is disposed near one end of the envelope 1@ within the por-tion 12. The beam source 15 comprises generally an electron emissiVe cathode 16, a control electrode 17, and a focusing and accelerating assembly 18. The construction and operation of the beam producing source 15 is that which is wellknown in the art, and in the particular embodiment shown typical operating voltages would be as follows. The cathode would be operated at a ground poten rtial while the control electrode 17 would lbe operated slightly negative wi-th respect to the cathode 16. The

focusing and accelerating assembly 18 may be operated at approximately 0.5 kilovolt positive with respect to the cathode.

'is sensitive to visible, ultraviolet or infrared radiation and includes a support member 31 which is of suitably transmissivevmaterial such as glass, or aluminum oxide lm.

A layer 32 isV disposed upon the surface. of the support member 31 (facing the assembly 15) and is of a suitable electrically conductive material transmissive of the input radiation, for example a very thin aluminum, gold, or stannous oxide film. Disposed upon the layer 32 is a layer 33 of suitable radiation responsive material such as antimony trisulphide, arsenic trisulphide or arsenic triselenide. A lead in 34 is provided to connect electrically the conducting layer 32 with a suitable source of potential 7 i). In fthe present embodiment, the conducting layer 32 is connected to a potential source of about l0 volts positive. An alternative structure would .provide that, with suitable precautions, the race plate 13 may serve as the support in lieu of member 31 and the layer 32 would be deposited directly onthe face plate 13 as is done in the usual vidicon.

Interposed lbetween the delico-ting plates 21 and the input screen 30 are a plurality of cylindrically-shaped conducting members 35, 36 and 37 which collectively fonn a collimating electron lens system. The elements 35, 36 and 37 are provided with suitable sources oi potential which may be approximately 0.5 kilovol-t, 1 kilovolt and 2 kilovolts, respectively. A decelerator grid v assembly 38 which may be at the same potential as the lens 37 is provided in close proximity to the input screen 30. The decelerator grid 38 serves to collect at least a portion of any secondaries emitted from the layer 33, as well as provi-ding, between it and the layer 33, a uniform electric eld in which the speed of the electron beam may be decreased without substantially changing its direction or diameter before it is incident upon the layer 33.

A second electron beam source 40 is disposed within the envelope 10 near to the deflection plates 21. This second beam source 40 comprises generally of an electron emissive cathode 41 and a control electrode 42. The electron source 40 is of the tluid beam type, that is, one which serves tordirect a beam of electrons uniformly over the entire surface of the layer 33. Flood guns are well known in the art of display storage Itubes and will not, therefore, be discussed in detail. In the present embodiment, the cathode 41 is provided with a suitable source or" potential 72, in the order of 30 volts negativel :L of voltage between the input screen 3i? and the second electron source 43 in order that electrons from this second source do not erase or destroy the image produced on the layer 33. This may be best understood with-reference to FIGS. 3 and 4. FIGS. 3 and 4 are graphical representations of typical secondary emission curves yfor various categories of materials, and in'which the target voltage with respect to the. cathode is plotted as the abscissa against net target current as the ordinate. From these curves, it is evident that if the potential between the ilood gun 46 and the target 33 is within the range represented by points A and B on the curves (B being the minimum portion of the curve which is due to secondary emission), that the secondary emission decreases as the voltage increases. Stated in another way, a greater number electrons are deposited on those portions of the target having the higher voltage. With this situation it is obvious that the electron beam will soon cause the entire target to assume a uniform potential near cathode potential which would result in an erasure of the information pattern thereon. On the other hand, if the potential between the iiood gun 4&1 and the target 3S is within the correct range, that is betweenI points B and C on the curves, it is evident that the rate of secondary emission rises as the voltage differential increases and hence no deterioration of the information pattern takes place within this latter range.

The slope of the curve may be considered as the plate resistance of the llooding beam. This resistance is a positive resistance of almost constant value between A and B and is a negative resistance of almost constant value betweenB and C. In an equivalent circuit, this plate resistance is in parallel with the target resistance. Treating a small area of the target, andthe flooding beam characteristic at this same area, as elemental resistances and considering them together with the elemental capacity of the target, the RC time constant ofV the target increases when the flooding beam plate resistance is negative and decreases when the plate resistance is positive. Since the magnitude of the ilooding beam current determines the plate resistance, the ilooding beam may be used to adjust the effective time constant to the best value for a particular set of operating conditions. Y

Positioned intermediate the electron gun 15 and the deflection plates 19 is an electron multiplier assembly indicated generally byV the reference character 5t). vThe multiplier assembly 5t) comprises a rst dynode 47 which is located coaxially with the electron beam produced by the means 15 and which has an aperture centrally located therein. The assembly 50 also includes a plurality of hollow toroidal-shaped electrodes 51, 52, 53 and 54 which are operated in the range of from 0.5 to 2.0 kilovolts and an output collector or electrode 55 which may beroperated at any potential of about 2.0 kilovolts. Positioned between the electron multiplier assembly 50 and the deilection plates 19 is an electrode 46 which may be in the form of an apertureddisc. The disc 46 is centrally 1ocated so that the primary electron beam from the electron gun assembly 15 passes through the aperture to be directed onto the layer 33.

The aperture within the electrode 46 is termed the selection aperture and provides selective collection of electrons returning from the target member 30. By selective collection is meant that only those electrons, either returned primary or secondarily emitted, emanating from a small incremental arca of the target 33 are permitted to pass to the electron multiplier assembly to be utilized as the output signal. t

The principle of selective collection can be applied by ,arranging the electron optics so that the return beam, modulated by the information which is on the target or retina 33, traverses the deflection region before being collected by the output electrode. Within the limitations of the abbe'rations of the electron optical system, this area can be made as small as desired by adjusting the size of the aperture of the electrode 46. The selective collection of electrons must come from an area that includes the reading beam spot. To facilitate this, the same deflection means may be used both to deflect the reading beam and to select the area of collection. Because of tolerances and aberrations it is impossible, as a practical matter, that the aperture Within the electrode 445 be of the same size as the reading beam spot and still include the return beam at all deliection angles. As a practical embodiment, the diameter of the selected area may be in the order of 1 percent of the diameter of the entire target or retina 33 and the aperture in the electrode in would be in the order of ten times that of the diameter of the reading beam spot. Under these conditions, that maximum portion of the tiood beam which is reflected and collected is proportional to the relative spot and total target areas and is only one part in ten thousand of that possible by the total flood beam. Thus, as the noise corrtributed by the fluctuation in the flood beam varies as the square root of this factor, it is only l% of the noise otherwise produced. However, since the whole of the reading beam signal is accepted by the aperture within the electrode 46, the signal-to-noise ratio is increased by 100. This technique of selective collection thus makes it possible to utilize a flood beam for either supplying the bulk of the dark current or enhancing the stored signal without degrading the output signal information with flood beam noise.

The manner in which the present invention serves to improve the signal-to-noise ratio of a pickup tube is best illustrated with reference to FiG. 2. The electron beam source 1S produces a primary or r ading beam ib which traverses the major length of the device and is, after being deflected by the deiiection plates 119, 2li and 2l and acted on by the collimating lens system 35, 3d and 37, directed onto the target Siti. A radiation image impingent upon the target 36 through the face plate i3 causes this target to have an incremental change of resistance so that a portion of the reading beam ib is utilized as the signal current. Also, a very small percentage of this reading beam current may provide the remainder of the dark current as the dood beam preferably supplies about 99% of the required dark current. The remainder of the reading beam is returned to be utilized as an output signal. Also ineluded in the return signal are secondarily emitted electrons occasioned by the primary beam ib plus those secondaries caused by the action of the flood beam. The sum of the return primaries and those secondaries emitted from the incremental area of the target on which the primary beam is impingent is represented in HG. 2 by ir. Because the primary beam is impingent upon the layer 30 at only a few volts energy and the return beam ir leaves the layer Btl also at only a few volts energy, the return beam if is acted upon by the same forces which initially acted upon the primary beam ib so that the path of ir follows, in reverse direction, substantially that, subject only t minor aberrations and inaccuracies of the focusing system, of the primary beam ib. As such it is permitted to pass through the aperture of the selection plate 46. This return signal is then incident upon .the electrode d'7 giving rise to secondary electrons in the electron multiplier system Si? to be collected on the output collector 55 in a manner Well known in the art to form the output signal.

Lines if and is of FIG. 2 represent one ray of the flood beam which is not incident upon an area of the target immediately adjacent to the primary electron beam. The flood beam portion if directed onto the target 3u in other than reading spot areas causes a reflected and secondary current iSec to be returned in the direction of the output circuit. However, as the current ec originates at a difierent point on the layer 3d it is acted upon by forces different from those which affect the primary beam ib. The return beam ir, is deflected by these forces in a different manner so as to be collected on other portions of the tube, for example, the deflection plates Ztl, 21 and 19. It is seen that as the sample flood beam portion if more nearly approaches the area of the primary beam, the current iSEG will more nearly follow the path of the return beam ir. As suc'n, the current isec is collected on the various tube elements until finally the apertured member d effects the final selection thereof.

lf the flood gun provides, for example, 99% of the required dark current, the reading beam can be reduced by the same amount while retaining the same value of the video output signal. However, the reading beam current noise is reduced by the square root of 10G or a factor of 10 times and the output signal-to-noise ratio improved by this same factor. The limit of this improvement is related to the spatial randomness of the flood gun electrons and the smallest practical reading beam size. It is expected that the limit may be such that the signal-to-noise ratio may be improved by a factor as high as ten times. This reduction in reading beam current noise results in a substantial improvement in the sensitivity of the camera tube.

The charge image on the layer 33 is destroyed by providing that the reading beam cathode supplies suthcient current to cause the layer 33 to reach cathode potential. Thus there is provided destructive read-out such as is Well known in the art.

While there has been shown and described what 'is at present considered to be the preferred embodiment of the invention, modifications thereto will readily occur to those skilled in the art. 4For example, it is evident that the iiood beam assembly could be replaced by a second scanning beam to scan coextensively, though not coincidentally with the primary scanning or reading beam.

It is not desired, therefore, that the invention be limited to this speciiic range as shown and described, and it is intended to cover in the appended claims: all such modifications as fall within the true spirit and scope of the invention.

We claim as our invention:

1. An image pickup tube comprising an evacuated envelope, a radiation sensitive target positioned within said envelope, said target exhibiting the property of producing a charge image in accordance with the radiation image thereon, said target being of a material having a resistance sui'liciently low to produce a dark current in the absence of an image, first electron `beam producing means positioned remotely from said target for directing an electron beam on said target, means including a source of potential between said first beam producing means and said target whereby said electron beam supplies the signal current for said target, collector means for receiving beam electrons rejected by said target, second electron beam producing means positioned to direct a beam of electrons on said taret to supply at least the bulk of the dark current, a source of potental connected between said target and said second beam producing means, and electron beam deflection means associated with said collector means to provide that only those electrons from the rst of said beams which are rejected by the target and those electrons emanating from the target in the immediate area of incidence of said first electron beam are collected on said collector.

2. An image pickup tube comprising an evacuated envelope, a radiation sensitive target positioned Within said envelope, said target exhibiting the property of producing a charge image corresponding to the radiation image thereon, said target being of a material having a sumciently low resistance to produce a dark current in the absence of an image, first electron beam producing means positioned remoteiy from said target for directing an electron beam on said target, means including a source of potential between said first beam producing means and said target to provide that said electron beam supply the signal current for said target, second electron beam producing means positioned to direct a beam of electrons on said target to supply the bulk of the dark current, a source of po- -ing only those beam electrons which are rejected and secondarily emitted by the target from a small area irnmediately adjacent the point of incidence of said first i electron beam on said target.

3. `An image pickup tube comprising an evacuated envelope, a radiationk sensitive target positioned within said envelope, said target exhibiting the property of producing a charge image corresponding to the radiation image thereon, said target being of a material having a suiiiciently low resistance to produce a dark current in the absence of an image, iirst electron beam producing means positioned remotely from said target for directing an electron beam of small area onto said target, means including a source of potential between said first beam producing j, means and said target to provide that said electron beam supplies the signal current for said target, second electron beam producing means positioned to direct a beam of electrons on said target to supply at least the bulk or" the -dark current, and a source of potential connected between envelope, said target exhibiting the property oi' producing a charge image corresponding to the radiation image thereon, said target being of a material having a sutliciently low resistance to produce a dark current in the absence of an image, iirst electron beam producing means positioned remotely from said target for directing an elec- -tron beam on said target, means including a source of potential between said -rst beam producing means and said target to provide that said electron beam supplies the signal current for said target, second electron beam producing means positioned to direct a beam of electrons on lsaid target to supply at least the bulk of the dark current, -a source of potential connected between said tar-get and said second beam producing means, said latter potential being of a value to provide that the electrons from the second beam producing means strike the target at an energy corresponding to that portion on the secondary emission curve of the target material at which there is an increase in secondary emission with'an increase in voltage, and means for selectively collecting only those beam electrons which are repulsed and secondarily emitted electrons from a small portion of the tar-get immediately adjacent the point of incidence of said first electron beam on said target.

5. An image pickup tube comprising an evacuated envelope, a radiation sensitive target positioned Within said envelope, said target exhibiting the property of producing a charge image corresponding to the radiation image thereon, said target being of a material having a suliiciently low resistance to produce a dark current in the absence of an image, iirst electron beam producing means positioned remotely from said target for directing an velectron beam on said target, means including a source of potential between said iirst beam producing means and.V

said target to provide that said electron beam supplies the signal current for said target, second electron beam producing means positioned to direct a beam of electrons on said target to supply at least the bulk of the dark current, `a source of potential connected between said target and said second beam producing means, said second electron beam being operative over an area on said target substantially greater than that instantaneously operative by said iirst electron beam, and means for selectively collecting only those beam electrons rejected and secondarily emitted from a small area immediately adjacent the point or incidence of said rirst electron beam on said target, said area not exceeding a value of l0() times the area instantaneously operative by said first electron beam.

6. An image pickup tube comprising an evacuated envelope, a radiation sensitive target positioned within said envelope, said target exhibiting the property` of producing a charge image corresponding to the radiation image thereon, said target being of a material having a resistance such that the product of target resistance and target capacitance is longer than the time between reading frames, iirst electron beam producing means positioned remotely from said target for directing an electron beam on said target, means including a source of potential between said v rst beam producing means and said target to provide that said electron beam supply the signal current for said target, second electron beam producing means positioned to direct a beam of electrons on said'target at an energy level such that the plate resistance in negative and the absolute value of this negative resistance is dependent upon the magnitude of the second electron beam current, 4and means for selectively collecting only those beam electrons rejected and secondarily emitted from a small Varea immediately adjacent the point of incidence of said first .45k

electron beam on said target.

References Cited by the Examiner UNlTED STATES PATENTS y2,652,515 `9/55 McGee 315-11 2,747,133 5/56 Weimer 315-ll 2,989,658 6/61 Palmer 315-10 X DAVID G. REDlNBAUGI-I, Primary Examiner.

ROBERT SEGAL, Examiner. 

1. A IMAGE PICKUP TUBE COMPRISING AN EVACUATED ENVELOPE, A RADIATION SENSITIVE TARGET POSITIONED WITHIN SAID ENVELOPE, SAID TARGET EXHIBITING THE PROPERTY OF PRODUCING A CHARGE IMAGE IN ACCORDANCE WITH THE RADIATION IMAGE THEREON, SAID TARGET BEING OF A MATERIAL HAVING A RESISTANCE SUFFICIENTLY LOW TO PRODUCE A DARK CURRENT IN THE ABSENCE OF AN IMAGE, FIRST ELECTRON BEAM PRODUCING MEANS POSITIONED REMOTELY FROM SAID TARGET FOR DIRECTING AN ELECTRON BEAM ON SAID TARGET, MEANS INCUDING A SOURCE OF POTENTIAL BETWEEN SAID FIRST BEAM PRODUCING MEANS AND SAID TARGET WHEREBY SAID ELECTRON BEAM SUPPLIES THE SIGNAL CURRENT FOR SAID TARGET, SECOND ELECTRON BEAM ELECTRONS REJECTED BY SAID TARGET, SECOND ELECTRON BEAM PRODUCING MEANS POSITIONED TO DIRECT A BEAM OF ELECTRONS ON SAID TARGET TO SUPPLY AT LEAST THE BULK OF THE DARK CURRENT, A SOURCE OF POTENTAL CONNECTED BETWEEN SAID TARGET AND SAID SECOND PRODUCING MEANS, AND ELECTRON BEAM DEFLECTION MEANS ASSOCIATED WITH SAID COLLECTOR MEANS TO PROVIDE THAT ONLY THOSE ELECTRONS FROM THE FIRST OF SAID BEAMS WHICH ARE EJECTED BY THE TARGET AND THOSE ELECTRONS EMANATING FROM THE TARGET IN THE IMMEDIATE AREA OF INCIDENCE OF SAID FIRST ELECTRON BEAM ARE COLLECTED ON SAID COLLECTOR. 